I want to start decreasing the body fat in my body, so been trying to work out what % and heart rate i should be exercising on the cross trainer in,

my heart rate resting is 72bpm, so were my calculations right of 125-165bpm is 65-85% which is the level i need to be at to burn fat optimally?

I went on cross trainer and did 18mins at 27kph, which was 126 bpm, tbh i couldnt possibly go any faster as i felt like i was hit by a train, i got to 10 mins and legs were like lead, but managed to get through that and eventually stopped at 18 mins ( 2 mins short of my 20 min goal )

Im planning on doing this 3 times 1 week and 4 times the next, basically every other day.

Can anyone give me any advice and let me know if i was right with calculations?

Sarah